The curing of bright-leaf (or flue-cured) tobacco is particularly energy intensive, due to the large amount of moisture which must be removed from the freshly harvested leaf. For example, to dry a typical tobacco barn containing 10,000 pounds of green tobacco leaves, one must remove as much as 8,500 pounds of water. Conventional tobacco curing methods are relatively inefficient, requiring an input of heat energy approximately three times that theoretically needed for vaporizing the moisture contained in the harvested leaf. The curing and drying of bright-leaf tobacco is conventionally carried out by the xe2x80x9cbulk curingxe2x80x9d method, in which tobacco leaves are loaded in a relatively compact mass in racks or in containers and placed inside of an enclosed curing barn, where a furnace circulates a forced flow of heated air through the tobacco leaves under prescribed conditions of temperature and humidity.
The curing process involves several distinct phases. Energy demand in the tobacco curing process differs at various phases during the curing cycle. The first phase, called yellowing, typically takes from about36 to 72 hours. During this phase, energy demand is relatively low, with the air temperature from about 95xc2x0 F. to about 105xc2x0 F. and with high air recirculation and relatively high humidity. After yellowing, the drying phase begins, and energy demand increases significantly as the curing air temperature is gradually increased up to about 165xc2x0 F. to about 170xc2x0 F. with low humidity during most of the drying phase and reduced air recirculation. These conditions fix the color and promote drying of the tobacco leaves. The thin, leafy portions of the tobacco dries first, and the thicker stem portions dry later. The total drying phase, including leaf and stem drying, requires about 3 to 4 days. The final phase is conditioning of the tobacco. When the cure is complete, the tobacco is generally dry and brittle, and requires the addition of sufficient moisture to soften it enough to allow it to be handled for removal from the barn. The total time for processing one xe2x80x9cbatchxe2x80x9d of tobacco is generally about 5 to 8 days, depending upon maturity of the freshly harvested leaf and other factors. Harvest of an entire crop generally occurs over a 5-7 week period, with successive batches of tobacco cured in the same barn.
In the past, tobacco was flue-cured: an open flame furnace was used to generate heat. A drawback of this system was that the resulting combustion gases were introduced, along with the hot air, into the barn containing the tobacco. These combustion gases can produce harmful compounds in the cured tobacco such as nitrosamines. Moreover, the hot air used to evaporate water from the tobacco leaves was usually exhausted. The exhaust air was still hot, and therefore large amounts of heat energy were wasted. Heat from exhaust air has typically not been recovered because the exhaust is undesirably wet, and conventional methods have not been able to separate the heat from the humidity.
Recently, heat exchangers have been added to prevent combustion gases from entering the barn, using an indirect firing system. However, most heat exchangers are only about 60 to 80 percent efficient at transferring heat from the furnace to the drying fluid, which is air in most cases. Thus, the technologies employing heat exchangers often contribute to even greater energy waste because the furnace must generate more heat than is actually used in the drying process. The need exists for a drying system which saves energy and allows for adjustments and control of the temperature, circulation, humidity of the drying air.
The invention is a drying system and a method for its use, the drying system having a drying chamber or container containing a product to be dried and a dryer unit. The dryer includes an inlet for providing the drying air, a drying air pathway, and a heating source, typically a furnace, for heating the drying air. The drying air flows through a supply route to the container, where the drying air dries the product and is converted into a return air. The return air flows through a return route back to the dryer. The return air flows through a return air pathway and a heat exchanger which intersects the return air pathway and the drying air pathway. The heat exchanger transfers heat from the return air to the drying air. The return air is exhausted through an outlet. In a preferred embodiment, the drying chamber is a tobacco barn, the product is tobacco, and the heat exchanger is a heat recovery heat pipe.